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Uncovering the secrets of millisecond pulsars: Why do they spin so fast?
Millisecond pulsars (MSPs) are pulsars with a rotation period of less than ten milliseconds. These stars have attracted the attention of astronomers with their amazing rotation speeds. They are detected in the radio, X-ray and gamma-ray parts of the electromagnetic spectrum. Millisecond pulsars are hypothesized to form by accreting matter from a companion star in a binary system, causing these old neutron stars to spin faster, and are therefore sometimes called recycling pulsars.
Millisecond pulsars are believed to be related to low-mass X-ray binary galaxies, and their formation and evolution remain important topics in astronomical research.
Origin of Millisecond Pulsars
Current theories generally believe that millisecond pulsars are related to low-mass X-ray binary systems. When the outer layer of the companion star in these systems exceeds the Roch limit, matter flows into the accretion disk of the neutron star, which causes the pulsar to rotate. The rate increases to several hundred revolutions per second. There may be evidence that standard evolutionary models cannot explain the evolution of all millisecond pulsars, especially some young millisecond pulsars with higher magnetic fields.
For example, PSR B1937+21 is a special case, and studies have shown that different millisecond pulsars may be formed by at least two different processes, and the nature of the second process remains a mystery. About 130 millisecond pulsars are now known to be found in globular clusters, which is consistent with the hypothesis of rotational acceleration during their formation, because such high star density increases the probability of pulsars interacting with giant companion stars.
The limit of pulsar rotation speed
In 1982, the first millisecond pulsar, PSR B1937+21, was discovered, which rotates at about 641 revolutions per second. The fastest pulsar discovered so far is PSR J1748-2446ad, which was discovered in 2004 and spins at 716 revolutions per second. Current models of neutron star structure and evolution predict that if the rotation speed exceeds 1500 revolutions per second, the neutron star will break up, and when the rotation speed exceeds 1000 revolutions per second, its energy loss will exceed the acceleration effect of the accretion process.
While candidates such as XTE J1739-285 show extremely high rotation rates, firm statistical support is still lacking.
Observations have shown that the rotation rate of quasars is very likely affected by gravitational radiation. For example, IGR J00291+5934 rotates at a speed of 599 revolutions per second and is one of the leading candidates for future detection of gravitational waves. Millisecond pulsars, due to their high-precision timing characteristics, can be used as a sensitive tool for environmental detection. They can detect celestial bodies as small as asteroids in the surrounding area and measure their masses.
Gravitational wave detection and pulsar timing
The existence of gravitational waves is a key prediction of Einstein's general theory of relativity. These waves arise from the large-scale motion of matter, ripples in the early universe, and the dynamics of space-time itself. As highly precise clocks, millisecond pulsars have rich application potential in celestial mechanics, neutron star seismology, and galactic astronomy.
The earliest proposals to use pulsars as gravitational wave detectors have been made possible by technological advances in the devices.
In the late 1970s, Sazhin and Detweiler proposed the idea of using pulsars as gravitational wave detectors by imagining an arm in space between the Earth and the distant pulsar. With the research of Foster and Backer in 1990, this method was further improved, especially for the application of high-stability millisecond pulsars.
Future Outlook
Future plans could further probe the gravitational wave background using precise timing techniques from pulsars. In 2023, NANOGrav published the results of 15 years of data, which preliminarily showed the existence of a gravitational wave background and measured the Hellings-Downs curve for the first time. This result provided strong experimental evidence for the detection of gravitational waves.
The study of millisecond pulsars not only enriches our understanding of the universe, but may also reveal many unsolved mysteries. So, what new discoveries will these mysterious and rapidly rotating celestial bodies bring us?
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